Easy REST calls from templates

As a template developer I want to easily make calls to REST endpoints from a template, so that I can use both local and external content in my web experiences.

Technical note: This is what the "restfn" does now.

Acceptance Criteria:

• REST calls are performed from the server so that credentials are not exposed and to avoid problems with CORS.
• I can use the response as content in my rendering.
• I can work with the JSON response in the same way that I would work with other content.
• I can use a REST Client, and if I do I can provide custom path fragments and query parameters to use in the REST call.